Sound recording method and apparatus



1949- H. R. IMO NTAGUE 2,477,640

SOUND RECORDING METHOD AND APPARATUS Filed June 27. 1944 2 i Fig.1

. PHASE I H SPLITTER 20 OSCILLATOR I 4 MODULATOR HOMER R. MONTA GUE 3 Patented Aug. 2, 1949 SOUND RECORDING METHOD AND APPARATUS Homer R. Montague, Washington, D. 0.

Application June 27, 1944, Serial No. 542,424

3 Claims.

(Granted under-the act of March 3, 1883, as

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to the art of sound recording, and particularly to an improved method of making the master record from which disc shaped phonograph records or transcriptions are commonly prepared, usually by the process known as pressing.

In the conventional method of producing masters, a disc of suitable material is placed on a turntable which is rotated at a fixed speed, and a groove is cut into the disc by a cutting head which is caused to travel parallel to a radius. The groove thus produced in the disc is a spiral, the sound energy being utilized to drive the stylus of the cutting head in such a way as to produce a radial modulation of the spiral groove, as is well known in the art. It is well known that any deviation from normal constant speed which occurs in the motion of the turntable produces frequency distortion in the reproduction; various schemes having been employed in order to maintain this speed as constant as possible, with moderate success. None of these schemes, however, is able to maintain the speed sufiiciently constant for extremely high fidelity recording.

Another source of difficulty in the making of I records or masters by the above described process is the great difficulty in obtaining cutting heads which do not introduce phase and amplitude distortion of the sound energy. This is partly because the mechanical impedance of the cutting mechanism varies with the frequency at which it vis driven, as well as with other factors, but there are other reasons for the presence of such types of distortion, all more or less attributable to the presence of mechanical masses in the system.

Accordingly, one object of my-invention is to provide a method of and means for producing sound records in which extremely constant recording speed may readily be obtained. Another object is to provide a sound recording system having no moving mechanical parts, and in which distortion attributable to the inertia and impedance of such parts is therefore entirely absent. These and other objects of my invention are obtained by utilizing a stationary sensitized record disc, the sound energy being used to control the position of a barn of cathode rays which normally describes a spiral path adjacent the surface of the disc, and which beam is converted to light which acts photochemically on the disc. Such 1 a disc, Whendeveloped photographically, is in fact amended April 30, 1928; 370 O. G. 757) a sound record itself, capable of being played by a suitable photoelectric phonograph. However, for the production of a master adapted to the process of pressing a plurality of duplicates, I utilize a disc in the form of a photoengraving plate, which, after exposure and development, can be etched to provide a physical sound groove similar to that produced by the conventional cutting process.

The accompanying drawings illustrate one embodiment of my invention, in which Fig. 1 is a diagrammatic view of a complete sound recording apparatus in accordance with my invention, while Fig. 2 is a plan view of a sound record produced with this apparatus.

Referring to Fig. 1 of the drawings, I designates a cathode ray tube of conventional design, having an evacuated envelope 2, provided at one end with a substantially flat surface 3. This surface is coated with phosphorescent compound as is usual in such tubes. The tube is provided with the usual cathode 4, grid 5, anode 6, horizontal deflecting plates I, I and vertical deflecting plates 8, 8, all as is well known. The cathode t is heated in the usual way by current from a transformer or other power source 9, while grid and anode voltages are obtained from any suitable D. C. source.

As stated, the surface 3 is substantially flat. It is more common in cathode ray tube practice'to make the viewing end curved, since this construe-- tion makes it easier to provide the necessary mechanical strength. In order to maintain a flat disc .closely adjacent the envelope, however, I prefer to make the surface 3 flat.

The recording material illustrated is a disc it of metal or other suitable material having a coating which is photographically sensitive. This disc is maintained substantially in contact with the fiat end 3 of tube I.

In order to cause the beam of cathode rays normally to describe a spiral path over the record material, I may use an oscillator l l whose output is sinusoidal and of a very low frequency, and the amplitude of which slowly decreases with time. Such oscillators are well known in the art, and the specific means employed to produce the output described is no part of my invention. The output voltage of this oscillator is split into two parts of equal amplitude but diiferin in phase angle by degrees, by means of a phase splitter diagrammatically shown at 12, the resulting two oscillating voltages bein applied respectively to the two pairs of plates 1, 7' and 8, 8. If the amplitudes of the two sine waves were constant, the

path traced by the cathode ray spot on surface 3 would be a circle, since the field causing deflection of the spot is the resultant of the two sine wave voltages applied to the deflection plates.

However, as stated above, the output of oscillator II has 'an amplitude which slowly decreases with time. Hence, :the amplitude :of each component applied to the deflecting plates also decreases with time, so that the curve traced by the cathode ray spot is actually a close spiral.

In order to produce the desired radial modu1a tion of the sound track, it is merely necessary to introduce the sound energy as an increment or decrement of the output amplitude of oscillator ll. formed by the modulator M, which may beof any of several well-known types. The sound energy to be recorded actuates microphone l which is connected to and controls modulator I 4, although it is of course possible ito control the modulator fromother translation devicesthanzmicrophones.

.l-n order to insure that the scanning process is not started prior .to the desired time, .a manual control .16 is provided at oscillator .11, operation .of which initiates the oscillations and the commencement oftheamplitude-decrease (amplitude attenuation) cycle.

.In-order that the upper frequency rangemay be faithfully recorded, it is desirable that the oath- .ode ray beam be focused accurately on the 'plane of the phosphorescent layer, and that the result- .ing luminous spot *be of very small size. It is also desirable to use sensitive material which :is very ficontrasty? in the photographic sense of the word, since such materials give a very sharp image and tend to minimize the effects of any diffusion which .may take place, such as that due to the size of the particles in the phosphor on the face 3 of tube 1|.

In the event that the radius-of disc ll is so-large compared to the distance from surface .3 to the virtual source of the cathode rays that it is diff].- -cult to keep the beam in focus for all values of radial deflection required, the focus may be corrected automatically by adjustin the voltage on grid 5 in accordance with'the amplitude attenuation controlling the radius of the .trace. This may be done, for example, by applying a voltage obtained from the attenuating means to the :an-

ode of the tube I, in addition to its normal potential, as illustrated diagranunatically by the connection in Fig. 1,.

After the recording process has been complete-d, the record disc :l-D is removed, developed photographically, and etched in ,a manner well known in the art of photoengraving. 'The etchin process is preferably chosen so as to leave thegroove edges with a polished surface, in order to reduce the surface noise which would other-wise be pro duced in pressings made therefrom.

It will be observed that my process produces directly a metal master suitable for use in producingstereotype negative matrices in the usual way, that is, by electroplating the master and .strippingoff the plated layer. However, it is unnecessary to render my master record electrically conducting prior to plating, since the original metal plate is itself an excellent conductor.

In the event that a very high polish is desired in the groove of a master prepared according to my invention, exceeding that obtained with the best available etching solutions, the master may be subjected to any :ofseveral well-known electrolytic polishingtechniques. These may, if dcsired, be applied prior to the removal of the re- This function I have shown as being perslst which remains on the unrecorded portions of disc Ill after the etching process. Such a polishing step is desirable where pressings especially free from background noise are required.

It is obvious that a record disc in produced in accordance with my invention-can be played back directly .on a conventional phonograph instead of, or in addition to, being used as a master for the production of matrices as used in the pressing process. Being of metal, my original record is of course 'a great deal harder and more rugged than materials ordinarily used for this purpose, In fact, :one of the advantages of my process is that the master may be played back directly for checking purposes priorto the making of matrices and pressings therefrom, without the loss of quality that occurs when ordinary cuttin materials are played even once prior to the making of the matrices.

The frequency of oscillator II and the rate of amplitude attenuation of its output depend upon the speed at which it is desired to reproduce the record and the desired pitch of the spiral. For example, to produce a record for playingat 578.26 revolutionszper minute (a standard speed for conventional phonographs), a frequency of 9 of a cycle per second is required, since onecomplete cycle represents one complete revolution of the cathode ray spot on disc in, and since very closely. If it is desired that the average pitch of the spiral groove be 112 lines per inch, which is a widely used standard, then the rate of amplitude attenuation of the outputof oscillator H must be such that after 112 cycles of the-output of oscillator I i, corresponding to 11 2 revolutions o'f'the cathode ray trace, the amplitude will drop 1/V volts, where Vis the voltage sensitivity of the cathode ray tube in inches per volt.

It will be observed that the constancy of the angular velocity of the cathode ray trace on record I-ll depends only upon the constancy of the frequency of the oscillator H, and not upon the linearity of its amplitude attenuation. Ex-

ceedingly constant speed may be obtained with ordinary types of oscillators such aspiezo-electric or magnetostriction oscillators, with suitable frequency dividers to reduce the frequency to the desired value. Since the rate of amplitude attenuation is not particularly critical, such attenuation may be produced, if desired, by a variable potentiometer driven by a 'low-power motor such as a small synchronous motor operated from the power mains. However, I have found that it is perfectly feasible to perform this function by a decay circuit incorporated in the oscillator itself.

My invention is obviously subject to many refinements that will occur to those skilled in the "art. For example, a manual or automatic 'amplitude attenuating device may be interposed between oscillator H and phase splitter 11-2, for rapidly changing the radius of the curve/traced on disc Ill. Such an arrangement would permit the production of the relatively high pitch starting and stopping grooves found on many recordings produced with conventional recorders.

I claim:

1. In a sound recording system, a cathode ray tube, means for supporting a sensitive record blank in a stationary position adjacent said tube, means for causing the luminous spot produced by said tube to trace anon-repetitious spiral path over a blank supported by said supporting means,

5 and means for modulating the motion of said beam, orthogonally to the general direction of movement of said beam, in accordance with the sound energy to be recorded.

2. In sound recording apparatus, the combination with a cathode ray tube provided with means for deflecting the cathode ray beam in two directions at right angles to one another, of a constant frequency oscillator, means for dividing the output of said oscillator into two components of equal amplitude and difiering in phase by 90, means for applying said two components to the deflecting means of said tube, means for varying the amplitude of the output of said oscillator slowly in a substantially uniform manner whereby the trace of said tube becomes a closely spaced, non-repetitious, multi-turn spiral, and means connected to said oscillator and said tube for simultaneously varying both of said components equally in amplitude in accordance with sound to be recorded, whereby a radial modulation of said spiral trace is accomplished, and means for holding a sensitive record blank in position to be impinged upon by the trace of said tube.

3. The method of making a disc-shaped sound record comprising providing a. disc of photosensitive material, producing a narrow beam of cathode rays, deflecting said beam electrically so as to trace a non-repeating closely-spaced, multi-turn spiral of substantial length upon the surface of said disc, modulating the position of said beam orthogonally to the general direction of said spiral at a rate corresponding to the frequency of sounds to be recorded and at an amplitude sufficiently small to prevent overlap between adjacent turns of said spiral, converting at least a portion of the energy of said beam into photo- 5 graphically actinic light prior to its impingement upon said surface, and thereafter developing said surface.

HOMER. R. MONTAGUE.

10 REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS The Cathode Ray Oscillograph In Radio Research, by Wall et al., His Majestys Stationery Oflice, London 1933, pages 85 to 91 inclusivg: 

